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Lathe spindle bearings and spindle material For lathe spindle bearings, i know one good option is to use two angular contact bearings on the far left, and a plain bronze taper bearing on the right. I was thinking about spreading the angular contact bearings apart to both ends of the headstock (while keeping a back-to-back arrangement and preload) and eliminating the bronze bearing. Is there any obvious reason why this would be problematic or inaccurate?
Bearings: ABEC 1 with 40 degree contact angle
For the spindle, i'm thinking about making it out of mild 1080 steel. If a plain bronze taper bearing is used, is 1080 steel compatible in terms of wear with the bronze bearing (assuming the spindle is polished to an optical finish with 0.3um and 0.05um polishing powder). Does anyone know what spindles are typically made out of (hardened and ground steel?)?
Heat buildup in the spindle lengthens it. The longer the distance between bearings, the greater the accumulative effect of heat, which then results in a change in preload.
I wouldn't use a plain bushing in a modern machine. Rather, a cylindrical roller bearing used at the outboard end can provide good centering without requiring any lengthwise preload consideration.
ABEC1....do you have to look hard for those?
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)
mackeym, I've seen near every kind of bearing used on lathe spindles. All have limitions and advantages.
If you use angular contact bearings, keep in mind the spindle shaft will grow with heat. The farther apart, the greater the growth and increase in preload. Also, an ABEC 1 is not the most accurate, it's the least. I'd rather see an ABEC 5 or better.
You will get many recommendations on material. Personally, I like 4140/50 or 4340. Your spindle should be hardened to improve its mechanical properties.
If you elect to use bronze bushing type bearings you MUST harden and grind the bearing surface of the spindle or you will wear the spindle as well as the bushing; much more hassle to replace the spindle than the bearing.
You might also consider tapered roller bearings (Timken brand has become almost a generic description). They can be had at the low-cost-not-so-accurate level as well as the high accuracy / expensive end.
DZASTR
If you choose bearings by TYPE and not true calculated bearing capacity versus applied load potential, you have no clue or assurance that the bearings will live let alone perform. Somewhere on the 'Zone, one of my prior rants about bearing load life calculations will go into further detail why this is the case.
The A/C's are there to control axial displacement and it is far away from the spindle for a reason - low RADIAL capacity. The bushed bearing is/was placed next to the chuck to accept the high radial loads inherent to lathe loading at that position.
Don't want the friction of a bushing? Consider a precision cyllindrical, and specifically one with a tapered bore. These are adjustable so as to set the internal clearance. THe use of fixed clearance garden variety cyllindricals have too much internal clearance for good finish and/or tolerance control.
You can run properly opposing tapers or A/C's, providing you account for axial growth under heat so as to NOT overload the bearings if the spindle grows with heat or unload the preload if the mounting position causes the opposite situation.
IF however, you chose to take a SWAG and run what you have giving no concern about load/life calculations, you can pretty much use anything. It may or may not work and nobody who REALLY knows ANYTHING about bearings will be able to say if it will work or how long it will live.
Caveat emptor.
ok. Thanks for the excellent tips and information. I'm going to go with 2 angular contact bearings (light preload) on the left side and one or two tapered roller bearings (or 2 angular contacts) on the right....and also think about thermal expansion some more and do some calculations.
One more question. To eliminate radial play between: 1. inner races and the spindle and 2. the outer races and headstock, I was thinking about making the parts a little bit too small/big, heating them up, push them together, so they lock together when they cool down. I would heat the parts to acceptable temperatures (not too hot), and also do some calcs to see what kind of expansion to expect. One fitting would be left loose to allow the spindle to expand axially. Are there any obvious problems or concerns with this?
Interference fit (press fit) can be a real PITA. I much prefer a modular means of locking the bearings in place... usually a cover plate for the outer race and shaft collar nuts for the inners. I preload using wavy disc springs.
I'm with DZASTR on the spindle material. Among those he listed, 4140 Prehard is my material of choice.
Is it possible to turn 4140 prehard or do you have to machine the unhardened metal and then have it hardened? If it's hardened after machining, will there be much dimensional change from the hardening process?
By the questions you are asking, you're a long ways from ripe yet
4140 prehardened is machinable with carbides quite readily. Drilling it for great distances is a bit more difficult due to the casual user not having the best equipment available for the job, but if you take your time, and use lots of coolant, you can drill with with HSS drills.
The bearings are usually a slight shrink fit on the spindle, and a very close 'tap in' fit in the housing. This is due to the simple fact that assembling/disassembling the contraption is a beast of a job if you do it otherwise.
If you go with two sets of angular contact or tapered rollers, one set should be constrained within a housing, but the other end should be able to float (axially lengthwise) otherwise you will still have undesirable preloading occurring due to differential heat expansion between the spindle and the housing. The spindle runs hotter than the housing in most cases.
A single tapered roller or angular contact bearing is useless. You would be better off with plain ordinary radial ball bearings on the outboard end of the spindle. If you have the bucks to spend, you can find ball bearings with lower than normal clearance, even precision grades might be available. However, the outboard bearing is just a steady basically, the front bearings do most of the work.
One question: how are you boring your headstock? Are you quite certain that you should not be starting off with the purchase of a lathe, new or used?
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)
mackeym, None of us here who are attempting to help you have enough information about your project. Give up more specifics and we can be more helpful. When it comes to bearings, pay attention to NC Cams, he's more up on the subject than most or all of the rest of us. He does this stuff 'cause he's dedicated. I do it 'cause I'm retired and bored stiff.
DZASTR
Thanks for more good info. Yes, i did think about redoing a used lathe. I was going to retrofit a micromark mini lathe. However, the only thing that would be left is a headstock, so i decided on building a larger one from scratch. (i'm going to use SHS25 rails for both axis's and ballscrews with AC brushless servos). I already have the ballscrews (C5 and better preloaded), one set of SHS (the other on the way), i have the motors and drives setup and working, and two 10:1 gear reducers (Bayside Stealth model).
For the headstock, i wanted to use a 12" x 12" x 2" granite surface plate for the base (i know that a special drill bit is needed for drilling mounting holes). I would then mount two 5"x5"x5" angle plates onto the surface plate giving two vertical supports. Then buy 1" x 8" x 18" flat ground stock (low carbon) and bolt this vertically onto the angle plates. But, before the ground stock is mounted, i will mill (CNC) out an area for the bearing outer races. I think that alignment of the holes for the outer races is important, so i plan on coming up with a way to ensure the alignment (after they're mounted) is better than 1mil.
And, is this going to be some sort of ultraspeed machine?
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)
haha, no I'm more interested in accuracy than speed (trying to get 1mil or better).
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